Constant rains increase almond disease threat

Persistent rains during almond bloom have kept California growers hopping — and they’ve kept bees that are important to pollination in their hives.

Wet weather and moderate temperatures have also increased the threat of fungal diseases, like brown rot blossom blight, shot hole and anthracnose, says Merced County UC Farm Advisor David Doll.

Right now, he says, brown rot is probably the biggest disease concern. It can grow in almost any temperature, but, “It reproduces much more quickly in the mid-50 to mid-60 degree range when there’s a lot of leaf wetness,” he says.

If not controlled, brown rot can kill flowers, reducing 50 percent or more. And, if the disease grows into the shoot, it can form cankers where the inoculum could survive to threaten next year’s crop.

Fungicides reduce disease incidence, “But, you need to use them wisely,” Doll says. “That means rotating among fungicide chemistries or FRAC (Fungicide Resistance Action Committee) groups to minimize the risk of developing resistant fungus strains, applying the fungicides properly, and understanding that each application offers no more than about 7 to 10 days of control, depending on the amount of rain.”

Often in a wet year, most growers spray twice to control brown rot — once at 5 percent to 10 percent bloom and the second time at 100 percent bloom. If rainy weather persists, even a third spray may be needed.

When rain washes off the fungicide, the effectiveness interval will be shorter. Even though it means more expense, Doll advises spraying fungicides to control brown rot, as well as shot hole, and jacket rot, for as long as rain is in the forecast.

“In a year with good weather conditions during bloom, you may be able to get by with just one spray at 100 percent bloom,” he says. “But this is definitely not that year.”

Once an orchard gets out of the pollination period, the risk of brown rot tails off, but if damp conditions continue, almond trees can start picking up scab and anthracnose.